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Chemical Properties of Monosaccharides

Hold on tight…

… here comes a quick review of organic chem.

Aldehyde and Ketone Reactions Con’t.�Formation of an AcetalFormation of an AcetalFormation of an AcetalFormation of an Acetal The

addition of alcohols to aldehydes produces an unstable hemiacetal intermediate that in-turn, produces an acetal. The alcohol here is a

reactant not a catalyst

Reactions, cont.�General formulas

Hemiacetal

Acetal

R R

Anomeric Carbon

• The hemiacetal is formed by the addition of an alcohol (R-O-H) by breaking the C = Ocarbonyl double bond and attaching the ether group (-O-R) to the carbonyl carbon and the hydrogen to the carbonyl oxygen.

• The unstable hemiacetal is then converted into an acetal by the substitution of the newly formed hydroxyl (-O-H) group by a new ether from a second alcohol (R-O-H), also producing water (H 2O).

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Reactions Con’t.�Formation of a Formation of a Formation of a Formation of a

KetalKetalKetalKetal The addition of alcohols to ketones produces an unstable hemiketal intermediate that in-turn, produces an ketal.

The alcohol here is a reactant not a catalyst

• The hemiketal is formed by the addition of an alcohol (R-O-H) by breaking the C = Ocarbonyl double bond and attaching the ether group (-O-R) to the carbonyl carbon and the hydrogen to the carbonyl oxygen.

• The unstable hemiketal is then converted into a ketal by the substitution of the newly formed hydroxyl (-O-H) group by a new ether from a second alcohol (R-O-H), also producing water (H 2O).

�Even though hemiacetals and hemiketals are relatively unstable, cyclical hemiacetals and hemiketals are commonly found because they are more stable than open chains.

Cyclic Structures of Monosaccharides�All monosaccharides with at least five carbon

atoms exist predominantly as cyclic hemiacetals and hemiketals.

�A Haworth structure can be used to depict the αααα and ββββ anomers of a monosaccharide.

�Anomers are stereoisomers that differ in the 3-D arrangement of groups at the anomeric carbon of an acetal, ketal, hemiacetal, or hemiketal group.

• Six-membered pyranose ring system

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• When the newly formed hydroxyl group forms below the ring structure, it is said to form the alpha anomer and when it forms above the ring structure, it is said to form the beta anomer.

• Anomers are sugars that differ in stereochemistry only at the hemiacetal or hemiketal carbon.

• Six-membered pyranose ring system

�Five-membered furanose ring system Naming Cyclic Monosaccharides

• Designate the anomeric form of the cyclic sugar structure by preceding the monosaccharide name with α or β

Classic Naming of Cyclic Monosaccharides

• Identify as 5 carbon ring (furanose) or 6 carbon ring (pyranose)

• Drop the –se from the straight chained monosaccharide name and replace with furanose or pyranose

• Designate the anomeric form by preceding the entire name with α or β

β-D-glucopyranose α-D-glucopyranose

α-D-fructofuranose β-D-fructopyranose

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Drawing Haworth Projections

• We have been representing cyclic monosaccharides using Haworth Projections, a standardized way of representing carbohydrate stereoisomerism in 3D space.

• Haworth projections are drawn directly from Fischer Projections simply by following the steps below.

1. Draw the correct Fischer projection for the monosaccharide.

• Note, the α or β is not determined by D or L.

Drawing Haworth Projections

2. Draw a primary pyranose or furanose ring structure depending on the monosaccharide.

3. Number the ring structure beginning with the carbon directly right of the oxygen atom in the ring (begin with the anomeric carbon).

• Remember, the last carbon is above the ring.

Drawing Haworth Projections

4. Assign hydroxyl group and hydrogens to all carbons other than the anomeric carbon following the following rule:

Hydroxyl groups pointing to the left in the fissure projection are position upward on the Haworth Projection and those hydroxyl groups pointing to the right are positioned downward.

5. Assign the acetal carbon a hydroxyl group corresponding to its anomeric designation ( β up, α down)

1.Try drawing the Haworth Projections for the following monosaccharides:

β-D-mannopyranose

α-D-xylofuranose

β-L-galactopyranose

Remember Benedict’s Test?�In the presence of aldehydes, Benedict’s

reagent (containing CuSO 4) produces a red copper precipitate.

Benedict’s reagent in

comparison to 2 glucose

solutions

Monosaccharide Reactions�Oxidation of a Reducing Sugar

The oxidation of a carbonyl group on a monosaccharide.

•Since all monosaccharides are in equilibrium with their cyclic form, they are all reducing sugars.•Benedict’s reagent is commonly used to test for the presence of reducing sugars:

Reducing sugar + Cu 2+ →→→→ oxidized compound + Cu 2Oblue orange-red

precipitate

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Monosaccharide Reactions, cont.�Esterfication The –OH groups of

monosaccharides can behave as alcohols and react with acids (especially phosphoric acid) to form esters.

This is an important reaction in the formation of Lipids studied in the next chapter

Monosaccharide Reactions, cont.�Glycoside FormationGlycoside FormationGlycoside FormationGlycoside Formation Cyclic

monosaccharide hemiacetals and hemiketals react with alcohols to form acetals and ketals, referred to as glycosides.

• The unstable hemiacetal is then converted into an acetal by the substitution of the newly formed hydroxyl (-O-H) group by a new ether from a second alcohol (R-O-H), also producing water (H 2O).

�The dehydration of the hemiacetal or hemiketal forms what is called a glycosidic linkage.

�A glycosidic linkage is an ether link between a carbohydrate (poly hydroxyl, or glycol compound) and another carbon molecule.